The mammalian circulatory system is comprised of a heart, which acts as a pump, and a system of blood vessels which transport the blood to various points in the body. Due to the force exerted by the flowing blood on the blood vessels, they may develop a variety of vascular defects. One common vascular defect known as an aneurysm is formed as a result of the weakening of the wall of a blood vessel and subsequent ballooning and expansion of the vessel wall. If an aneurysm is left without treatment, the blood vessel wall gradually becomes thinner and damaged, and, at some point, may be ruptured due to a continuous pressure of blood flow. Neurovascular or cerebral aneurysms affect about 5% of the population. In particular, a ruptured cerebral aneurysm leads to a cerebral hemorrhage, thereby resulting in a more serious life-threatening consequence than any other aneurysm, as cranial hemorrhaging could result in death.
Cerebral aneurysms may be treated by highly invasive techniques which involve a surgeon accessing the aneurysm through the cranium and possibly the brain to place a ligation clip around the neck of the aneurysm to prevent blood from flowing into the aneurysm.
A less invasive therapeutic procedure involves the delivery of embolization materials or devices into an aneurysm. The delivery of such embolization devices or materials may be used to promote hemostasis or fill an aneurysm cavity entirely. Embolization devices may be placed within the vasculature of the human body, typically via a microcatheter, either to block the flow of blood through a vessel with an aneurysm through the formation of an embolus or to form such an embolus within an aneurysm stemming from the vessel. A variety of deviceable coil embolization devices are known. Coils are generally constructed of a wire, usually made of a metal (e.g. platinum) or metal alloy that is wound into a helix. The coils of such devices may themselves be formed into a secondary coil shape, or any of a variety of more complex secondary shapes. Coils are commonly used to treat cerebral aneurysms but suffer from several limitations including poor packing density, compaction due to hydrodynamic pressure from blood flow, poor stability in wide-necked aneurysms and complexity and difficulty in the deployment thereof as most aneurysm treatments with this approach require the deployment of multiple coils.
Coils are usually delivered through a microcatheter, inserted through the groin for instance, as seen in FIGS. 1A and 1B. They are often inserted into the aneurysm via a pusher mechanism that relies on electrolytic erosion or a mechanical mechanism to detach the coil from the pusher. The microcatheter commonly tracks a guide wire to a point just proximal of or within the desired site for occlusion. The coil is advanced through the microcatheter and out the distal end so to at least partially fill the selected space and create an occlusion as seen in FIG. 1C. Once a coil is deployed at a desired site, occlusion results either from the space-filling mechanism inherent in the coil itself, or from a biological response to the coil such as a thrombus formation, or both. The space-filling mechanism of the coil may be either based upon a predetermined secondary geometry, or may be based upon random flow characteristics of the coil as it is expelled from a delivery sheath lumen. Coils may be inserted into the microcatheter in stretched, relatively linear conformations, but expand to larger, secondary memory shapes upon exiting the microcatheter's internal lumen.
In treatment of a cerebral aneurysm using coil embolization, about 20% cases do not require additional ancillary devices. But, particularly in the case of a wide neck cerebral aneurysm with a large orifice or a cerebral aneurysm with a large neck-to-fundus ratio, it is necessary to insert a stent or aneurysm exclusion device into a parent blood vessel, as seen in FIG. 1D, to cover a neck of the cerebral aneurysm so as to prevent migration of a coil that fills the aneurysm. Stents with aneurysm neck bridging devices that significantly occlude the flow of blood into the aneurysm may be used alternatively to coils or to supplement the embolization of the aneurysm. However, stents must remain both a low enough density to collapse into a conformation deliverable through a microcatheter and flexible enough to navigate the tortuous cerebral blood vessels. Additionally, it can be difficult to rotationally position such devices to precisely cover the defect. Covered stents or stent-grafts, comprising a sleeve of polymeric material around the stent lumen could be used to occlude the aneurysm but pose the risk of inadvertently occluding small perforator vessels proximate to the aneurysm. Stents also may provide a site for thrombus formation, with the resulting risk of embolization and stroke.